1. Field of the Invention
The present invention relates to moving beam laser scanners, and more particularly, to microelectromechanical systems (MEMS) that function as a scanning mirror for a laser scanner.
2. Description of Related Art
Optical imaging systems or beam scanners utilize a beam of light that is swept across a target from which information is to be obtained. These devices are commonly used to decipher data symbols printed on objects in order to identify the objects. A conventional bar code symbol typically comprises a one-dimensional or two-dimensional pattern of vertical bars of various widths separated by spaces of various widths. The term "symbology" is used to describe the unambiguous rules specifying the way data is encoded into the bar and space widths. Because the bar and space elements have different light reflecting characteristics, a scanner can convert the bar code symbol into an electrical signal by analyzing the light reflected from the symbol. The electrical signal can then be decoded to recover an alphanumeric representation of the symbol that identifies the object. Bar code symbols of this nature are now in common usage in various types of applications, such as inventory control, point of sale identification, or logistical tracking systems.
The bar code scanner typically uses a light source that is scanned across the bar code field. Because the bar code symbol is often disposed on the object to be identified, it is desirable for the scanner to be included in a hand held or portable device so that the scanner can be brought to the object. A bar code scanner may include an internal electromechanical system that automatically articulates the light source back and forth at a high rate to scan across the bar code field. Such moving beam scanners usually employ either a helium-neon or solid-state laser as a light source. The scanning motion is provided by rotating or oscillating mirrors inside the scanner, which can achieve a typical scan rate of approximately 40 scans per second. Such moving beam handheld scanning devices are advantageous because they require little operator skill and are capable of effectively reading suboptimum quality symbols by employing a large number of scanning attempts in a short period of time.
One drawback of current scanning devices is that the rotating or oscillating mirrors are large, complex, and often involve electromagnetic coils or oscillating motors along with many moving parts. This results in high cost and poor reliability. Furthermore, because the mirrors are large with a substantial amount of mass, there is a brief delay required between an operator's scan prompt and the actual scan. This delay is caused by the inertia associated with the mirror and its moving parts and is reflected in the time required to bring the mirror from a state of rest to the required scan rate. The delay will be readily detected by an operator who will notice the slow starting scan and will perceive this as a sluggish response. The delay will also result in an operator working less efficiently when performing many scan operations.
There are various fundamental limitations associated with reducing the size of conventional rotating or oscillating mirrors. For example, as the mirrors, magnets, and coils are reduced in size, their inertia is also reduced. A restoring force may be employed for the oscillating scan components by using metal springs, torsion elements, or the like, but as the size and inertia is reduced, the resonant frequencies tend to exceed the scan rate. Also, as a spring element is shortened, so is its range of travel, thus elastic limits may be approached well before the spring flexes sufficiently for its intended use. This results in fatigue and early failure and may be susceptible to unwanted vibrations, especially in portable hand held applications. Additionally, standard methods of construction for the conventional mirrors have limits in terms of the size or scale of a design. Thus, there are limitations to the degree of size reduction for conventional rotating or oscillating mirrors and their associated motors, coils, springs, and other required parts.
One area of science that offers significant size advantages, while overcoming the limitations of reducing conventional parts, is microelectromechanical systems (MEMS). MEMS, or the related field of microoptoelectromechanical systems (MOEMS), refers to systems that combine electrical and mechanical components, including optical components, into a package that is physically very small. These systems are generally fabricated using integrated circuit fabrication techniques or similar techniques such as surface micromachining or bulk micromachining. Various sensors and actuators can be built including engines, transmissions, transducers, resonators, and mirrors that are measured in terms of microns. The degree of complexity depends on the number of movable levels or planes that the fabrication technique provides. For example, a two-level system comprises one mechanical level and a stationary ground plane/electrical plane. Each additional level allows an additional mechanical level. By the third level, rotating gears and mirrors can be built and, by the fourth level, transmissions and hinged pop-up mirrors are possible. A common material used is polycrystaline silicon (polysilicon), because of its inherent strength and that it is directly compatible with modern IC fabrication. The devices offer good reliability and durability with tests performed showing over 4 billion cycles and speeds over 300,000 revolutions per minute. Thus, MEMS offer a way to reduce the size of scanning mirrors and accompanying hardware far beyond that possible by conventional methods.
There are many drawbacks concerning current implementations of optical scanners utilizing MEMS devices. For example, the MEMS device may be limited to scanning in only one direction. Also, the method of oscillating the MEMS device may utilize springs, combdrives, or other types of gears and mechanical sliders that add mass and complexity.
Accordingly, it would be desirable to provide a MEMS device that could serve as a scanning mirror for an optical imaging system. The MEMS device would provide a substantially reduced size than mirrors of conventional design and also offer improvements over current scanners using MEMS mirrors. This advantage would provide faster response times and overcome the inertia limitations of prior art. In addition to it being smaller and lighter, the MEMS device offers advantages in terms of being more precise and cost effective due to advantageous manufacturing processes similar to those of microelectronics. Furthermore, the MEMS device would provide increased reliability and lower cost due to the complete system being on a single silicon chip.